MRI is an imaging method which magnetically excites nuclear spin of an object placed in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on MR signals generated due to the excitation. The above-described MRI means magnetic resonance imaging, the RF pulse means a radio frequency pulse, and the MR signal means a magnetic resonance signal.
Here, an RF (Radio Frequency) coil is a coil device which applies an RF pulse to nuclear spin inside an object by, for example, supplying a coil with an RF pulse electric current and receives generated echo signals as MR signals. RF coils are classified into the following three types in terms of transmission and reception: a transmit-only RF coil, a receive-only RF coil, and a dual-purpose RF coil used for transmitting of RF pulses and receiving of MR signals. In addition, RF coils are also classified into a whole body type and a local type.
Since MR signals emitted from an object are weak, it is preferable to receive MR signals at a position as close to the object as possible. Thus, various types of the local RF coils, each of which is shaped so as to fit each part of a human body, are used depending on an imaging part.
In MRI, multi-channel system is developed in an acquisition system of MR signals. The above-described channel means each of pathways of MR signals outputted from each of coil elements and inputted to an RF receiver of an MRI apparatus. The number of channels is set to equal to or smaller than the input reception number of the RF receiver.
If the number of cables between an RF coil and a control side of an MRI apparatus such as an RF receiver side increases due to development of the above-described multichannel system, hard-wiring becomes complicated. Thus, wireless RF coils which wirelessly transmit/receive data between an RF coil and a control side of an MRI apparatus are developed.
In an MRI system configured to digitize MR signals and wirelessly transmit the digitized MR signals, a control-side clock signal of an MRI apparatus to be used as a reference and a coil-side clock signal inside a wireless RF coil are synchronized with each other in some way. This is because even if two oscillators, one of which is in the wireless RF coil and another of which in the control side of the MRI apparatus, output nominally the same frequency, two frequencies outputted by the respective two oscillators do not completely match each other due to individual differences. Thus, even if the phases of the respective two oscillators in the wireless RF coil side and the control side are matched each other at a certain time point, the phases of the respective two oscillators gradually shift from each other unless processing of synchronizing the coil-side clock signal with the control-side clock signal is continued.
When a synchronization signal is wirelessly transmitted from a control side of an MRI apparatus to a wireless RF coil side, an electromagnetic wave used for the synchronization signal must conform to the Radio Law and/or Regulations. However, Law or Regulations concerning radio transmission are different from country to country. Thus, the same transmission frequency and modulation system as those permitted in a certain country cannot always be used in another country.